1. Field of the Invention
The present invention relates to the field of radio telecommunications. More particularly, the present invention relates to calculating bit error rate estimates from control signals, including, but not limited to pilot signals, and measuring therefrom the link quality in a radio telecommunications system.
2. Background Information
In radio telecommunications, bit error rate (BER) estimates can provide valuable link quality information. BER is actually the frequency at which bit errors occur in signals, typically during transmission over the air interface. Large BERs may have a detrimental effect on signal quality, such that reducing BER is highly desirable.
Transmission rate and BER are two factors that may be considered in determining link quality and there exists a direct relationship between these two factors. Typically, a higher transmission rate results in a higher BER. Conversely, a lower transmission rate results in a lower BER. However, in the interest of increasing both signal quality and transmission rate, lowering transmission rate to obtain a lower BER is not a desirable option.
Besides transmission rate, there are other conditions that can precipitate a high BER. Other such conditions include, for example, high interference levels, low signal strength levels, and fading.
BER estimates may be calculated in any of a number of different ways. For example, one method for estimating BER is to re-encode the output of the channel decoder and compare the re-encoded output of the channel decoder with the input to the decoder. Typically, these BER estimates are only produced during transmission periods, which may, in turn, increase transmission delay. Since certain signals are highly sensitive to transmission delay, particularly voice signals, any method that might increase transmission delay is not desirable.
Typically, telecommunications systems transmit control information in addition to data information, for example, during periods of silence on a speech link. Thus, one way to avoid the above-identified problem associated with estimating BER during transmission periods would be to use control information rather than data to estimate BER.
Control information often includes known information symbols, for example, pilot symbols. Pilot symbols are used to estimate such things as time delay, gain, and phase for each of a number of received signals associated with a given transmission channel. Together, these time delay, gain and phase estimates form what is known as the channel impulse response.
Many code division multiple access (CDMA) based systems employ RAKE receivers. CDMA is a form of spread-spectrum communication where a transmitted signal is spread over a wide frequency band within the communication channel. Generally, a RAKE receiver collects and combines the energy arriving along resolvable components of the received signal (e.g., multipath channels associated). The signal energies from the several propagation paths as received from several fingers are combined or “raked” together by the RAKE receiver. A RAKE receiver can be employed to measure BER using, for example, control signals, such as pilot symbols.
RAKE receivers are well known to those skilled in the telecommunications art. Two articles illustrating the use of RAKE receivers in communication systems are “Performance Comparison between Time-Multiplexed Pilot Channel and Parallel Pilot Channel for Coherent RAKE Combining in DS-CDMA Mobile Radio”, by Abeta et al., pages 1417-1425 (1998) and “Channel Estimation Filter Using Time-Multiplexed Pilot Channel for Coherent RAKE combining in DS-CDMA Mobile Radio” by Andoh et al., pages 1517-1525 (1998). Additionally, U.S. Pat. No. 5,329,547 entitled “Method and Apparatus for Coherent Communication in a Spread-Spectrum Communication System” discloses a method for encoding and decoding in a spread spectrum system to facilitate coherent communications.
In order to combine the energy associated with each resolveable component of a signal, the RAKE receiver must generate a channel estimate ci for each RAKE finger, as one skilled in the art and readily appreciate. The channel estimate ci for a given RAKE finger of a RAKE receiver is expressed in accordance with equation (1) below:                               c          i                =                                            ∑                              k                ∈                                  k                  p                                                      ⁢                                          r                                  i                  ,                  k                                            ⁢                              a                                  i                  ,                  k                                *                                                          N            p                                              (        1        )            where it is assumed that the symbols are grouped in slots. FIG. 1 illustrates an exemplary slot 104. As shown, slot 104 includes a portion 102 that contains one or more pilot symbols ri,j and a data portion 103, where the position of a pilot symbol ri,j within slot i is denoted by j, wherein Np represents the number of pilot symbols in portion 102 of slot i, and wherein kp represents the number of positions j allocated for those symbols within portion 102 of slot i. In equation (1), a*i,k represents the complex conjugate operator associated with the transmitted pilot symbol ri,ki. The bit sequence associated with the channel estimate ci can then be compared to a known bit sequence. The BER can be derived based on this comparison.
There are disadvantages associated with deriving BER using control signal information such as pilot symbols. Particularly, when a received pilot symbol is used for both channel estimation and for BER estimation, the BER estimate is biased. Biasing occurs when there is a systemic deviation or variation of a value from a reference value. The reason for this is that part of the channel estimate which originates from the symbol causes a real, positive fractional component of the transmitted symbol to be added to the estimate of the received symbol when the received symbol is corrected to account for the influence of the channel. Accordingly, BER is underestimated and therefore it does not accurately reflect the true BER.
Equation (2) represents a relationship that may be used to derive the estimated contribution of pilot symbol rij for the channel estimate associated with the one RAKE finger, where wc is a real, positive weighting factor associated with the previous channel estimates C, and wi,j is a real, positive weighting factor for the channel estimate ci.                                           a            ^                                i            ,            j                          =                              r                          i              ,              j                                ⁢                                           ⁢                                                    (                                                                            w                                              i                        ,                        j                                                              ⁢                                          c                      i                                                        +                                                            w                      c                                        ⁢                    C                                                  )                            *                                                      w                                  i                  ,                  j                                            +                              w                c                                                                        (        2        )            
Equation (3) demonstrates how the received symbol contribution âi,j, associated with a given RAKE receiver finger, for the jth symbol in slot i, may be estimated. Those skilled in the art will appreciate that equation (3) may be derived by substituting equation (1) into equation (2).                                           a            ^                                i            ,            j                          =                              r                          i              ,              j                                ⁢                                                    (                                                                                                    w                                                  i                          ,                          j                                                                    (                                                                        ∑                                                      k                            ∈                                                          k                              p                                                                                                      ⁢                                                                              r                                                          i                              ,                              k                                                                                ⁢                                                      a                                                          i                              ,                              k                                                        *                                                                                              )                                        /                                          N                      p                                                        +                                                            w                      c                                        ⁢                    C                                                  )                            *                                                      w                                  i                  ,                  j                                            +                              w                c                                                                        (        3        )            
When the jth symbol contributes to the channel estimate, the bias can be isolated and the bias contribution estimated. This is shown by the second term, (ri,jr*i,jai,j)(wi,j/Np)/(wi,j+wc), in equation (4) below.                                                                                           a                  ^                                                  i                  ,                  j                                            =                            ⁢                                                                    r                                          i                      ,                      j                                                        ⁢                                                                                    (                                                                                                                                            w                                                                  i                                  ,                                  j                                                                                            (                                                                                                ∑                                                                                                            k                                      ∈                                                                              k                                        p                                                                                                              ,                                                                          k                                      ≠                                      j                                                                                                                                      ⁢                                                                                                      r                                                                          i                                      ,                                      k                                                                                                        ⁢                                                                      a                                                                          i                                      ,                                      k                                                                        *                                                                                                                              )                                                        /                                                          N                              p                                                                                +                                                                                    w                              c                                                        ⁢                            C                                                                          )                                            *                                                                                      w                                                  i                          ,                          j                                                                    +                                              w                        c                                                                                            +                                                                                                      ⁢                                                (                                                            r                                              i                        ,                        j                                                              ⁢                                          r                                              i                        ,                        j                                            *                                        ⁢                                          a                                              i                        ,                        j                                                                              )                                ⁢                                                                            w                                              i                        ,                        j                                                              /                                          N                      p                                                                                                  w                                              i                        ,                        j                                                              +                                          w                      c                                                                                                                              (        4        )            As indicated, the transmitted symbol ai,j in the second term of equation (3) is multiplied by a real, positive factor, as the receive symbol ri,j the only complex valued factor, is multiplied by its own complex conjugate.
Despite the removal of the bias, there remains additional disadvantages. If the above channel estimate is computed based on only one pilot symbol, and without any averaging with previous estimates, the first term of equation (4) will be equal to (0). This computation also makes it possible to use channel estimation for decoding of adjacent symbols. However, the demodulated pilot symbol the channel estimate is based on will always have the same phase shift as the transmitted pilot symbol.
In another case, the channel estimate is computed based on more than one pilot symbol or averaging over previous estimates, every pilot symbol is affected by the channel estimate contributions of the other pilot symbols. The problem is then reduced, but not eliminated. It would be useful to develop a method that accurately determines BER and link quality and overcomes some of the disadvantages mentioned above.